Formation of porous silicon nanostructures by metal-assisted chemical etching

Silicon nanostructures are promising materials for devices in fields of nanoelectronics, optoelectronics, energy conversion and storage, bio- and chemical sensors. There are many methods to fabricate Si nanostructures. Metal-assisted chemical etching (MACE) is a simple and effective method allowing to control various parameters of Si nanostructures (cross-sectional shape, diameter, length). MACE can be used to make higher surface‑to‑volume ratio structures, straight and well-defined pores or wires with diameters from 5 nm to 1 µm. Further formation of PS structures by the MACE method is studied.

The formation process of PS structures by the MACE method consists of two steps. On the first one, Ag nanoparticles are formed by immersion deposition on the surface of Si wafer. Ag was selected because of its high catalytic activity. On the second step, the Si wafer with Ag nanoparticles is immersed in the aqueous solution of HF and H₂O₂.

Fig. 1 presents surface view images of Ag deposited on the Si surface from 1 mM AgNO₃ solution by the immersion method. Sizes of Ag nanoparticles deposited on the surface Si are in the range of 100-300 nm. The distance between the particles is 200-300 nm (Fig. 1a). Fig. 1b shows that Ag is deposited as the quasi-continuous film.

Figure 1. SEM surface view images of Si wafer with the Ag nanoparticles deposited from the aqueous AgNO₃ solution with (a) 0.5 M HF and (b) 5 M HF for 30 min.

Let us consider a mechanism of Ag deposition on the Si surface. The Ag⁺ reduction reaction has a positive potential that allows to take electrons by Ag⁺ ions from the solutions. Si is oxidized while Ag⁺ is reduced to Ag⁰. It is obvious that prolonged exposure of the Si wafer in the aqueous AgNO₃ solutions leads to formation of the continuous layer of SiO₂ which prevents the contact of reagents. Therefore, the reduction of the metal is stopped.

In HF-containing solutions, SiO₂ is etched and Ag can deposit on the Si surface again. Thus, increasing concentration of HF in the aqueous AgNO₃ solution leads to an increase of the deposition rate and formation of the larger Ag particles that form the quasi-continuous film (Fig. 1b). Ag nanoparticles deposited from solution with 0.5 M HF (Fig. 1a) form dense chains, and the particle density is more than the density of the sample with Ag nanoparticles deposited from solution containing 5 M HF (Fig. 1b).

Fig. 2 presents SEM images of the surface (a, c) and cross-section (b, d) of the PS structures. The Si wafer has been etched in the aqueous solution containing 5 M HF and 0.3 M H₂O₂ for 60 min after the deposition of Ag from the aqueous 1 mM AgNO₃ solution containing 0.5M HF (Fig. 2a) and 5 M HF (Fig. 2c). Due to the higher catalytic activity of Ag, Si is predominantly oxidized at the contact of Ag nanoparticles with the Si surface. SiO₂ under Ag nanoparticles is etched by HF and Ag nanoparticles go down vertically into the Si wafer.

Figure 2. SEM top (a, c) and cross-section (b, d) images of Si wafers after the Ag immersion deposition from 0.5 M HF (a, b) and 5 M HF (c, d) solutions for 30 min followed by MACE for 60 min.

From SEM images of cross-sections (Fig. 2b,d), it can be seen that the structure resembles an array of Si nanowires with the thickness approximately 8‑12 µm which are vertical to the Si surface. However, the SEM top images of the surface show that the structure is PS with crystallites of 100-500 nm. There is practically no single nanowires (Fig. 2a), whereas in Fig. 2c the structure is a Si nanowires or nanobelts of 200-300 nm width congregated in the bundles. Both of the formed structures are ordered. Thus, increasing of the concentration of HF in the Ag deposition solution leads to formation of large Ag particles by which Si nanowires or nanobelts are produced in the etching process.

The Fig. 3 shows that thickness of the porous layer depends on the etching time linearly. 

Figure 3. Dependence of the thickness of the porous layer on the etching time.

The effective method of PS formation by the two-step MACE was presented. This method allowed to fabricate ordered PS structures (e.g. nanowires and nanobelts). Increasing of the concentration of HF from 0.5 M to 5 M in Ag deposition solution leads to the formation of large Ag particles which can provide formation of silicon nanowires or nanobelts in the etching process. The thickness of porous layer depends linearly on the etching time.